GE MiCOM P40 Technical Manual page 205

P446SV
The following are some of the main features and requirements for a POR scheme:
The scheme requires a duplex signalling channel to prevent possible maloperation if a carrier is keyed for an
●
external fault. Because the signalling channel can be keyed for faults external to the protected zone, it is
vital that they are only received by, and acted upon by, the intended recipient. A simplex channel cannot
assure this.
● A POR scheme may be more advantageous than a PUR scheme for the protection of short transmission
lines. This is because the resistive coverage of the Zone 2 elements may be greater than that of the Zone 1
elements (in the case of Mho elements)
● Current reversal guard logic prevents healthy-line protection maloperation for high speed current reversals
that can be experienced on double circuit line applications, which can be caused by sequential opening of
circuit breakers.
If the signalling channel fails, basic distance scheme tripping remains available.
●
The POR logic is:
Send logic: Assert signal if Zone 2 element picks up.
●
● Permissive trip logic: Trip if the Zone 2 element operates AND the Channel Aided signal is received.
The POR scheme also uses the reverse looking zone 4 as a reverse fault detector. This is used in the current
reversal logic and in the weak infeed echo feature, shown dotted in the figure below:
P446SV-TM-EN-1
Chapter 8 - Carrier Aided Schemes
173